1. Field of the Invention
The present invention is related to perpendicular magnetic recording mediums, particularly, related to a perpendicular magnetic recording medium containing a perpendicular recording layer having a saturation magnetization Ms of not more than 400 emu/cc and a lattice constant c of not more than 4.07 xc3x85.
2. Description of the Related Arts
Generally, in a perpendicular magnetic recording medium applied to a magnetic tape, a magnetic card and a magnetic disc, a perpendicular magnetic recording layer is, at least, formed on a non-magnetic substrate shaped in the form of a tape, card or disc. Recently, the perpendicular magnetic recording medium has been given an attention to, and has been actively developed because of capability of its high density recording on the perpendicular magnetic recording layer by perpendicularly recording a recording signal such as a video signal, a sound signal and a digital data thereon.
As this kind of the perpendicular magnetic recording layer, a Coxe2x80x94Cr system alloy thin film has been vigorously studied because of its strong perpendicular anisotropy and its excellent magnetic recording characteristics. The crystalline structure of the Coxe2x80x94Cr system alloy thin film has a hexagonal closest packed structure (hcp) and it is possible to obtain a strong perpendicular magnetic anisotropy by approximately orienting a c-axis, which is a magnetization easy axis, perpendicular to a surface of the thin film.
On the other hand, a high coercivity is required for the recording medium to obtain a high output level and a high S/N as well as a high density recording. Especially, in the case of the perpendicular magnetic recording medium, a high perpendicular coercivity is important to maintain a high security of recorded signals other than a high output level and a high S/N, otherwise, the recorded signal is easily degraded due to thermal relaxation because a squareness ratio Rs=Mr/Ms (a ratio of a perpendicular remanent magnetization Mr to a saturation magnetization Ms) is decreased since a demagnetizing field generated in a perpendicular direction corresponding to a thickness of the thin film is very high due to an extremely small thickness of the thin film compared with other dimensions in inner plane directions of the thin film.
Thus, in order to enhance the coercivity Hc of the perpendicular magnetic recording medium, there are proposed some methods in the prior arts. As one of the examples, there has been studied a method where a third element such as Ta, Zr, Nb and Pt is added to Coxe2x80x94Cr system alloy. Thereby, it is possible to obtain a coercivity of nearly 3,000 Oe by adding Pt of 10 at %, and about 2,600 Oe by adding other elements other than Pt thereto.
Further, as another example, there has been studied another one for forming a multi-layer with a superlattice such as Pt/Co, Pd/Co, Pd/CoCr, or Pt/CoCrTa. In any multi-layers with the superlattice, it is possible to obtain a high coercivity Hc of not less than 3,000 Oe and a squareness ratio of not less than 0.9, resulting in that excellent magnetic recording characteristics are apparently obtained due to an exchange interaction of the multi-layer.
However, in the former example to enhance the coercivity of the perpendicular magnetic recording medium in the prior art, i.e., the method of adding Pt of no less than 10% to Coxe2x80x94Cr system alloy, there are problems that a cost performance is degraded because of a high cost of the element Pt, and a saturation magnetization Ms becomes not less than 410 em/cc. When the saturation magnetization Ms is too high, a recording medium noise increases, resulting in a degradation of S/N of the recording medium.
Further, in the case where an element other than the element Pt is added to Coxe2x80x94Cr system alloy, it is possible to obtain a high coercivity Hc of not less than 2,500 Oe along with a saturation magnetization of nearly 300 emu/cc. However, in this case, it is necessary to raise a temperature of the substrate to not less than 400xc2x0 C. when the perpendicular magnetic film is formed thereon. This causes a problem of a selective limitation that a high heatproof material has to be employed as the non-magnetic substrate. In addition, it requires a large power source and a heatproof apparatus to heat the non-magnetic substrate and further production processes for heating and cooling the non-magnetic substrate.
On the other hand, in the later example to enhance the coercivity of the perpendicular magnetic recording medium in the prior art, when the multi-layer with the superlattice is formed on the non-magnetic substrate, the exchange interaction thereof is apt to causes a recording medium noise, resulting in a degradation of S/N. Further, the production process becomes complicated because of the multi-layer.
Accordingly, it is a general object of the present invention to provide a perpendicular magnetic recording medium in which the above disadvantages have been eliminated.
It is a more specific object of the present invention to provide a perpendicular magnetic recording medium comprising, at least, a perpendicular magnetic recording layer having a hexagonal closest packing (hcp) structure of which c-axis is perpendicular to a surface of the perpendicular magnetic recording layer, on a non-magnetic substrate, wherein the perpendicular magnetic recording layer has a saturation magnetization Ms of not more than 400 emu/cc, and a lattice constant c of the hcp structure is not more than 4.07 xc3x85.